Forskolin and isoproterenol effect discrete responses on epidermal growth factor induced DNA synthesis in aortic smooth muscle cells.
Defining the mechanisms regulating the proliferation of vascular smooth muscle is necessary to better understand the pathogenesis of atherosclerosis and hypertension. In the present investigation, we examined the effects of incubation with forskolin or isoproterenol on the proliferation of cultured rat aortic smooth muscle cells. Forskolin, a direct activator of adenylate cyclase, and isoproterenol, a beta-adrenergic agonist, increased intracellular cyclic AMP levels in a concentration-dependent manner, subsequent to a 5-min exposure. Isobutylmethylxanthine at 100 microM attenuated epidermal growth factor stimulated DNA synthesis by 35% without affecting intracellular cyclic AMP levels. Forskolin dose-dependently augmented this inhibition. In contrast, a 24-h exposure of cells to isoproterenol resulted in a biphasic effect on growth factor stimulated thymidine incorporation. Both forskolin and isoproterenol attenuated thymidine incorporation to the same degree up to 12 h poststimulation, the onset of S phase. By 16 h poststimulation, [3H]thymidine incorporation in smooth muscle cells treated with isoproterenol was significantly enhanced by 50%, whereas forskolin treatment continued to attenuate DNA synthesis by 40%. Somewhat surprisingly, this disparity in effect on DNA synthesis was evident in spite of heterologous desensitization to rechallenge by either forskolin or isoproterenol subsequent to a 24-h incubation with either drug. These results suggest that the isoproterenol enhancement of epidermal growth factor stimulated DNA synthesis in rat aortic smooth muscle cells may be cyclic AMP independent.